Basic and Bedside Electrocardiography, 1st Edition (2009)

Chapter 5. Heart Rate and Voltage

The ECG Paper

· The standard electrocardiogram (ECG) is recorded at a paper speed of 25 mm per second. The voltage is calibrated so that 1 mV gives a vertical deflection of 10 mm.

· ECG paper: The ECG paper consists of parallel vertical and horizontal lines forming small squares 1 mm wide and 1 mm high. Every fifth line is highlighted and is darker than the other lines, thus defining a larger square of five small squares vertically and horizontally. An example of an ECG is shown in Figure 5.1.

o Width: The width of the ECG paper represents time. Every millimeter or one small block is equivalent to 0.04 seconds, because the ECG records with a paper speed of 25 mm/second. Every highlighted line containing five small squares is equivalent to 0.20 seconds.

o Height: The height represents voltage. Because the height is standardized to give a deflection of 10 mm per mV, every small square is equivalent to 0.10 mV. The calibration marker is routinely recorded at the beginning or end of a 12-lead tracing (Fig. 5.1).

Figure 5.1: The Electrocardiogram (ECG) Paper. The ECG paper is divided into small squares. The width of the smallest square is 1 mm, which is equivalent to 0.04 seconds. The height of the smallest square is 1 mm, which is equivalent to 0.10 mV. When a 12-lead ECG is obtained, a calibration signal is routinely recorded such that 1.0 mV gives a deflection of 10 mm.

Calculating the Heart Rate

· There are several methods of calculating the heart rate from the ECG.

o Using the large boxes: The heart rate, expressed in beats per minute (bpm), can be calculated by counting the number of large boxes between two R waves (Fig. 5.2).

· Using the small boxes: Another method of calculating the heart rate is by counting the number of small boxes between two R waves. This is the most accurate method when the heart rate is regular and fast (Fig. 5.3).

o Using 3-second time markers: A third method of calculating the heart rate is by using the 3-second time markers, which are printed at the top margin of the ECG paper. The distance between the time markers is 3 seconds. The heart rate is calculated by counting the number of QRS complexes within 3 seconds and multiplied by 20. The first complex is the reference point and is not counted (Fig. 5.4).

o Using 6-second time markers: If the heart rate is irregular or very slow (Fig. 5.5), a longer time interval such as 6-second time marker or even 12-second time marker is chosen. The heart rate is calculated by counting the number of QRS complexes within 6 seconds and multiplied by 10. If 12 seconds are used, the number of complexes is multiplied by 5, to obtain the heart rate per minute.

o Not all ECG papers have 3-second time lines. A 3-second time line, however, can be created by counting 15 large blocks in the ECG paper. Similarly, a 6-second time line can be created by counting 30 large blocks.

o Using commercially available heart rate sticks: Several commercially available heart rate meters can be used to calculate heart rates. The meter is placed on the ECG rhythm strip and the heart rate is read directly from the meter stick as shown (Figs. 5.6 and 5.7). Using a heart rate meter stick is a very convenient way of measuring heart rates. Unfortunately, they are not always available when needed.

o Using a heart rate table:When the heart rate is regular, a heart rate table can be used for calculating heart rates. When calculating heart rates, it is more convenient to use the larger boxes for slower heart rates and the smaller boxes for fast heart rates if the heart rate is regular. Note that the same heart rate can be obtained by using the formula 300 divided by the number of big boxes or 1,500 divided by the number of small boxes, as explained earlier. If the heart rate is irregular as in patients with atrial fibrillation, a 6- or 12-second rhythm strip is more accurate (Fig. 5.8).

Figure 5.2: Calculating the Heart Rate Using the Large Squares. The heart rate can be calculated by the formula 300 ÷ the number of large squares between two R waves. Thus, if there are 5 large squares between 2 QRS complexes, the heart rate is 60 beats per minute (300 ÷ 5 = 60).

Figure 5.3: Calculating the Heart Rate Using the Small Squares. Using the small squares, the heart rate can be calculated by the formula 1,500 ÷ the number of small boxes between two R waves. Thus, if there are 5 small squares between 2 QRS complexes, the heart rate is 300 beats per minute (1,500 ÷ 5 = 300).

Figure 5.4: Calculating the Heart Rate Using the 3-Second Time Markers. There are seven complexes within the 3-second time line. The heart rate is 7 × 20 = 140 beats per minute. Note that the first QRS complex is the reference point and is not counted.

Figure 5.5: Calculating the Heart Rate Using the Time Markers. Because the heart rate is very slow, a longer interval is measured and two 3-second markers (6 seconds) are used. There are four complexes within the 6-second time line. Thus, the heart rate is 4 × 10 = 40 beats per minute.

Figure 5.6: Heart Rate Meter Stick Using Two Cardiac Cycles. An example of a heart rate meter stick is shown. This heart rate stick uses two cardiac cycles to measure the heart rate. Two QRS complexes are measured starting from the reference point, which is identified by an arrow on the left side of the meter stick. The heart rate is read directly from the meter stick and is 76 beats per minute.

Figure 5.7: Heart Rate Meter Stick Using Three Cardiac Cycles. This particular meter stick uses three cardiac cycles to calculate the heart rate. Three cardiac cycles are counted starting from the reference point, which is at the left side of the meter stick. The heart rate is read directly from the meter stick and is 76 beats per minute.

Figure 5.8: Figuring the Heart Rate. When the rhythm is regular, the heart rate can be calculated by measuring the distance between two QRS complexes using the large boxes (upper portion of the diagram) or the small boxes (lower portion of the diagram). The larger boxes are more convenient to use when the heart rate is <100 beats per minute, whereas the smaller boxes are more accurate to use when the heart rate is faster and >100 beats per minute.

Figure 5.9: Tall Voltage. Two sets of electrocardiograms (ECGs) were obtained from the same patient representing the precordial leads and a lead II rhythm strip. (A) The ECG was recorded at normal calibration. Note that the amplitude of the QRS complexes is very tall with the calibration set at normal standard voltage (10 mm = 1.0 mV). (B) The ECG was recorded at half standard voltage (5 mm = 1.0 mV). Note that the amplitude of the QRS complexes is smaller, the ECG is less cluttered, and the height of the QRS complexes is easier to measure. The calibration signals are recorded at the end of each tracing and are marked by the arrows.

ECG Voltage

· Voltage: The height or amplitude in the ECG paper represents voltage. The calibration signal is routinely printed at the beginning or end of the 12-lead recording (Fig. 5.9) and is standardized so that 1 mV gives a deflection of 10 mm. If the QRS complexes are too small (low voltage) or too tall (tall voltage), the standardization can be doubled or halved accordingly by flipping a switch in the ECG machine.

· Tall voltage: The voltage of the QRS complex is increased when there is hypertrophy of the left ventricle. This is further discussed in Chapter 7, Chamber Enlargement and Hypertrophy. The voltage in the precordial leads is also normally taller in young individuals, especially African American males, in patients who are thin or emaciated, and in patients with mastectomy, especially of the left breast.

· Low voltage: Excess fat, fluid, or air does not conduct impulses well and will attenuate the size of the complexes. The distance between the heart and the recording electrode will also influence the voltage in the ECG. Thus, the complexes in the limb leads are smaller than the complexes in the precordial leads because the location of the limb electrodes is farther from the heart. Fluid around the heart, lungs, abdomen, body, or extremities as well as obesity will also attenuate the size of the complexes. The low voltage may be generalized or it may be confined to the limb or frontal leads.

o Low voltage in the limb leads: Low voltage confined to the limb leads indicates that not a single QRS complex measures 5 mm (0.5 mV) in any of the frontal or limb leads. The voltage in the chest leads is normal.

Figure 5.10: Low Voltage with Electrical Alternans. Twelve-lead electrocardiogram showing generalize low voltage. Note that not a single QRS complex measures 5 mm in the limb leads or 10 mm in the precordial leads. In addition, there is also beat-to-beat variation in the size of the QRS complexes because of electrical alternans (arrows). The presence of a large pericardial effusion was verified by an echocardiogram.

Electrical Alternans

In electrical alternans, there is a beat-to-beat variation in the size of the QRS complexes usually by >1 mm. An example of alternating voltage of the QRS complex resulting from significant pericardial effusion is shown in Figure 5.10.

Calculating the Heart Rate and Measuring the Voltage

ECG Findings

· Heart rate: The number of heartbeats per minute can be counted accurately using the ECG.

· Voltage: The voltage of any wave in the ECG can also be measured by its amplitude.

Mechanism

· Heart rate: The QRS complex represents activation of the ventricles, which causes the heart to pump blood to the different parts of the body. The heart beat per minute can be counted by palpating the radial pulse or more accurately by counting the number of QRS complexes in the ECG.

· Voltage: The height of the different complexes in the ECG depends on a number of factors, which can either increase or decrease their amplitude. Increased ventricular mass and close proximity of the recording electrode to the origin of the impulse will enhance the voltage. On the other hand, the presence of fat, fluid, or air and a longer distance between the origin of the impulse and the recording electrode will attenuate the voltage in the ECG.

Clinical Implications

· Heart rate: Included as one of the vital signs in the evaluation of any patient is the heart rate. When the patient is on a cardiac monitor, the heart rate is displayed together with the ECG rhythm. The heat rate can also be obtained very accurately in a recorded ECG. This can be done rapidly by measuring the distance between two R waves when the heart rate is regular. If the heart rate is irregular, a longer rhythm strip is needed for a more precise reading. Note that the heart rate obtained by ECG is more accurate than the pulse rate obtained at bedside because not all the impulses recorded in the ECG may be strong enough to generate a cardiac output that is palpable as a pulse, especially in sick patients who are hypotensive or in heart failure or when the patient has an irregular rhythm. In these patients, the pulse rate is not always equal to the heart rate.

o Regular heart rate: When the rate is <100 bpm, the larger boxes are more convenient to use. When the rate is >100 bpm, the smaller boxes are more convenient and more accurate to use. The distance between two QRS complexes in large or small boxes is used for counting the ventricular rate and the distance between two P waves for counting the atrial rate.

§ Large boxes: The heart rate per minute can be calculated using the formula: 300 ÷ number of large boxes between two R waves. The formula is based on the following information.

§ Standard ECG paper speed = 25 mm per second or 1,500 mm per minute. Because one large box = 5 mm, ECG paper speed is 1,500 ÷ 5, or 300 large boxes per minute.

§ Heart rate per minute = 300 ÷ number of large boxes between two QRS complexes.

§ Small boxes: The heart rate per minute is obtained by dividing 1,500 by the number of small boxes between two R waves. The formula is derived from the following information:

§ Standard ECG paper speed = 25 mm per second or 1,500 mm per minute.

§ Heart rate per minute = 1,500 ÷ number of small boxes between two QRS complexes.

o Irregular heart rate: When the heart rate is irregular (atrial flutter or atrial fibrillation), a longer interval should be measured to provide a more precise rate. A 3-second time line can be created if it is not marked in the rhythm strip. A 3-second interval is equal to 15 large boxes.

§ If a 3-second time interval is used, multiply the number of QRS complexes by 20.

§ If a 6-second time interval is used, multiply the number of complexes by 10.

§ If a 12-second time interval is used, multiply the number of complexes by 5.

§ Note that the first QRS complex is used as a reference point and is not counted.

· Voltage: Tall voltage in the ECG suggests that there is increased mass of the right or left ventricle. This is further discussed in Chapter 7, Chamber Enlargement and Hypertrophy. Decreased voltage of the QRS complex occur when transmission of the cardiac impulse to the recording electrode is diminished and are frequently seen in patients who are obese or patients with chronic pulmonary disease, pleural or pericardial effusions, generalized edema, hypothyroidism, or when there is infiltrative cardiomyopathy, such as in amyloidosis, causing reduction in the number of myocytes in the ventricles and atria.

o Electrical alternans: Alternating voltage of the QRS complex can occur when there is significant pericardial effusion, which allows the heart to swing in a pendular fashion within the pericardial cavity. When the heart moves closer to the chest wall, the QRS complex becomes taller. When it is pushed further away from the chest wall by the next beat, the QRS complex becomes smaller. The alternating size of the QRS complex is best recorded in the precordial electrodes especially V2 to V5 because these leads are closest to the heart. Electrical alternans because of pericardial effusion can occur only if the pericardial effusion is large enough to allow the heart to swing within the pericardial cavity. Alternation of the QRS complex because of pericardial effusion is a sign of cardiac tamponade. Electrical alternans can also occur even in the absence of pericardial effusion when there is abnormal conduction of the electrical impulse in the ventricles alternating with normal conduction. It can also occur during supraventricular tachycardia or when there is severe myocardial ischemia. Electrical alternans can involve any wave of the ECG including P waves, QRS complexes, and T waves.